Primary wing structure with strain gauges

Abstract

The invention relates to a primary airfoil structure on which mechanical stress is to be detected. A strain gauge is used at a measuring point, which is designed for a material with a coefficient of thermal expansion α. B is designed, while the material of the wing primary structure differs from the intended material and has a coefficient of thermal expansion α T exhibits. According to the invention, the strain gauge is designed and applied such that an apparent strain (10) results from the different coefficients of expansion α. B , α Tand at least partially compensate for a deviation (9) of the value of a target temperature profile (3) of the strain gauge (19, 20) at a target temperature (8). Preferably, the strain gauge designed and applied in this way is used in a special Wheatstone bridge circuit, whereby compensation strain gauges can also be used. The design according to the invention allows for a reduction of a measurement error (11) at a target temperature (8) that deviates from a reference temperature (4) of the strain gauge, or within a temperature range (12).

Description

TECHNICAL AREA OF INVENTION

[0001] The invention relates to a primary wing structure with strain gauges. A primary wing structure is specifically designed to absorb the mechanical stresses acting on the wing of an aircraft and to hold the wing to a fuselage. The primary wing structure can, for example, be a beam, girder, or tube extending in the main direction of the wing. Secondary wing structures (e.g., wing boxes or any attachment components) can be attached to and held by the primary wing structure. For example, the wing incorporating the primary wing structure can be a solar-electric manned or unmanned aircraft designed for high altitude and long range.

[0002] For the development and design of the aircraft, the wing and the wing primary structure and / or for the operation of the wing, the wing primary structure and the aircraft, it may be of interest to know the mechanical stresses acting on the wing primary structure.

[0003] The invention relates to a primary airfoil structure on which an acting mechanical stress can be measured by means of strain gauges. STATE OF THE ART

[0004] The publication Voß, A., Soal, K., Sinske J., Meier, D., Niemann, S., Nickle, J., and Hanke M., “Pre-Test of a Light-Weight CFRP Wing Segment of a High Altitude Platform for In-flight Load Measurements Based on Strains,” presented at the German Aerospace Congress, Bremen, 2021, https: / / doi.org / 10.25967 / 550028, proposes equipping a primary wing structure with strain gauges to measure mechanical stresses. Four strain gauges are used, connected to form a Wheatstone bridge. To capture different stresses, three different arrangements of the four strain gauges on the primary wing structure are proposed, enabling the measurement of mechanical stresses resulting from shear force, bending moment, and torsional moment.The publication already raises the question of how to prevent the influence of strain gauge measurements on temperature fluctuations of the surface of the primary airfoil structure. It describes a problem that when strain gauges of the Wheatstone bridge are arranged on both the upper and lower surfaces of the primary airfoil structure, the two surfaces can exhibit very different temperatures if one side is heated by sunlight while the other remains unheated. Other causes of temperature changes can include unevenly acting cold winds, varying angles of solar radiation, and similar factors.To avoid measurement errors due to these temperature differences, it is proposed to cover the strain gauges, thereby at least reducing the influence of solar radiation and cold winds. A cover made of 5 mm thick foam with an aluminum strip is proposed, the aluminum strip serving to reflect solar radiation.

[0005] The website https: / / www.hbm.com / de / 6725 / temperaturkompensation-bei-dehnungsmessstreifenmessungensind Information from a strain gauge manufacturer on avoiding undesirable temperature influences on strain gauge measurements can be found here. The suggestion is to use self-compensating strain gauges. It is described how a temperature change in the structure to which the strain gauge is attached leads to an apparent strain measured by the strain gauge due to the material's coefficient of thermal expansion α, even though no actual mechanical stress is present.Apparent strain does not occur when self-compensating strain gauges are used. In these gauges, the temperature behavior of the strain gauge is matched to the temperature behavior of the material of the structure to which it is applied, thus eliminating apparent strain. This is achieved by matching the thermal expansion coefficient of the strain gauge to that of the structure. Since the thermal expansion behavior of the strain gauge is non-linear with temperature, the self-compensating strain gauge is adjusted so that no apparent strain is present at room temperature (20°C). The non-linear component constitutes a residual error, which is zero at room temperature and exhibits a (local or absolute) maximum. A residual strain error occurs with increasing or decreasing the temperature relative to room temperature.At room temperature, the residual error is zero, while it increases with increasing deviation from room temperature. The residual strain error can be approximated using a polynomial, which is determined by the strain gauge manufacturer and provided to the customer to perform computational compensation for larger temperature changes compared to room temperature. The publication also describes the use of a Wheatstone bridge for evaluating the measurement signals of self-compensating strain gauges when connected as a half-bridge or full-bridge, as well as the use of a three- or four-wire connection to minimize the influence of cable resistance.

[0006] On the manufacturer's other website https: / / www.hbm.com / de / 10083 / temperaturkompensation-bei-dms-viertelbrueckenanwendungen provides further information on the computational compensation of a temperature-related measurement error.

[0007] For further information on strain gauges and their wiring for measuring different mechanical stresses on structures, please refer in particular to the websites https: / / www.me-systeme.de / de / grundlagen / dehnungsmessstreifen / schaltungsvarianten, https: / / www.ni.com / de / shop / data-acquisition / sensor-fundamentals / measuring-strain-with-strain-gages.html and https: / / www.ni.com / en / shop / data-acquisition / sensor-fundamentals / measuring-strain-withstrain-gages / how-is-temperature-affecting-your-strain-measurement-accuracy-.html referenced. TASK OF INVENTION

[0008] The present invention is based on the objective of proposing a primary airfoil structure which enables the measurement of a mechanical stress of the primary airfoil structure by means of a strain gauge at a target temperature that deviates from a reference temperature of the strain gauge with a reduced measurement error. SOLUTION

[0009] The object of the invention is achieved according to the invention by the features of the independent claim. Further preferred embodiments of the invention can be found in the dependent claims. DESCRIPTION OF THE INVENTION

[0010] The primary airfoil structure according to the invention has a measuring point that measures a coefficient of thermal expansion α. Texhibits. A strain gauge is attached to the measuring point of the primary wing structure. The strain gauge has a target temperature profile, which is determined at a reference temperature, in particular room temperature, and when attached to a material with a coefficient of thermal expansion a. B has a (local or absolute) maximum and is in particular zero, while the target temperature profile exhibits negative values ​​that increase with the distance from the reference temperature when deviating from the reference temperature.

[0011] While the state of the art teaches the selection of self-compensating strain gauges, where the material of the measuring point is the determining material of the strain gauge, so that the coefficient of thermal expansion α T the coefficient of thermal expansion α BThe invention proposes that a strain gauge is specifically selected whose material of determination differs from the material at the measuring point, so that the coefficient of thermal expansion α also corresponds to the material of determination. B of the material being determined, the coefficient of thermal expansion α T the primary structure of the wing deviates from this. This has the seemingly negative effect that the actual temperature gradient of the strain gauge at the reference temperature, when applied to the measuring point of the primary structure of the wing, is not zero and / or does not exhibit a local maximum. Rather, the actual temperature gradient at the reference temperature deviates from zero, and a slope of the actual temperature gradient at the reference temperature may occur that does not deviate from zero.

[0012] According to the invention, it is proposed that the target temperature range of the strain gauge is matched to the material of the primary wing structure in such a way that, surprisingly, at a target temperature at which the measurement on the primary wing structure is to be carried out and which deviates (possibly significantly, for example by more than 10°C, more than 20°C, more than 30°C or even more than 40°C) from the reference temperature, a measurement error is at least reduced. For this purpose, the target temperature range of the strain gauge is selected in such a way as to match the material of the primary wing structure that - an apparent strain ε S , which result from the different coefficients of thermal expansion α B and α T and the resulting different strains ε T the supporting structure at the target temperature and ε DMS of the DMS at temperature, and - to at least partially eliminate a deviation in the value of the temperature coefficient of the strain gauge at the target temperature compared to the value of the temperature coefficient of the strain gauge at the reference temperature.

[0013] The aforementioned apparent deterioration resulting from the selection of an "incorrect" strain gauge, designed for a different material than the material of the measuring point on the primary wing structure, surprisingly leads, according to the invention, to a reduction in the measurement error at the target temperature. The invention is also based on the understanding that the previously explained apparent strain ε S On the one hand, and on the other hand, the deviation of the temperature coefficient of the strain gauge at the target temperature compared to the value of the temperature coefficient of the strain gauge at the reference temperature have different signs, so that these can partially cancel each other out.

[0014] The invention offers numerous possibilities for the extent to which the apparent strain and the aforementioned deviation are eliminated. For one proposed aspect of the invention, the apparent strain ε differs. S and the deviation of the temperature coefficient of the strain gauge at the target temperature from the temperature coefficient of the strain gauge at the reference temperature by a maximum of 50%, a maximum of 40%, a maximum of 30%, a maximum of 20% or even a maximum of 10%.

[0015] The invention also offers numerous possibilities for selecting the target temperature at which the at least partial elimination of apparent strain and deviation is designed. One proposed design is for a target temperature in the range of 40°C to 100°C, preferably in the range of 60°C to 90°C, 70°C to 85°C, 0°C to 80°C, -40°C to 70°C, or -70°C to 80°C. These target temperature ranges correspond to temperatures that can occur on the upper surface of a wing as a result of heating from intense solar radiation and / or cooling at high altitudes.

[0016] The selection of the strain gauge according to the invention for a primary wing structure can be carried out with any arrangement and interconnection of the strain gauge and / or to detect any mechanical stresses.

[0017] For a first variant of the invention, the primary structure of the wing is designed to measure a bending stress of the primary structure of the wing.

[0018] Preferably, in this case, a strain gauge is arranged in the area of ​​a measuring point that is subjected to tensile stress due to the bending stress being measured. A further corresponding strain gauge is then arranged in the area of ​​another measuring point that is subjected to compressive stress due to the bending stress. The two strain gauges are thus located on opposite sides of the neutral axis of the wing primary structure subjected to bending, particularly in the area of ​​the upper and lower surfaces of the wing primary structure. In this case, a Wheatstone bridge circuit can be used, which has two parallel voltage divider branches. In the first voltage divider branch, a first resistor and a fourth resistor are arranged in series, with a first bridge point located between the first and fourth resistors.In the second branch of the voltage divider, a second resistor and a third resistor are connected in series. A second bridge point is located between the second resistor and the third resistor. A measuring bridge connects the first bridge point to the second bridge point. In this configuration of the Wheatstone bridge circuit, the strain gauge forms the first resistor, while the second strain gauge forms the second resistor. The first and second resistors are located on the same sides of their respective bridge points in both branches of the voltage divider. This configuration of the strain gauges in the Wheatstone bridge circuit leads to increased measurement accuracy when determining the tensile and compressive stresses resulting from bending, and thus to increased measurement accuracy when determining the acting bending moment.

[0019] In this embodiment of the Wheatstone bridge circuit, a further aspect of the invention allows for the use of a compensation strain gauge and a further compensation strain gauge. The [further] compensation strain gauge is arranged in the region of the [further] measuring point, but oriented such that it is not subjected to tension or compression due to the bending stress of the primary airfoil structure. Preferably, the [further] compensation strain gauge is oriented with its measuring direction parallel to the bending axis of the bending stress. The compensation strain gauge then forms the fourth resistor of the Wheatstone bridge circuit, while the further compensation strain gauge forms the third resistor of the Wheatstone bridge circuit.

[0020] Within the scope of the invention, it is possible that the compensating strain gauge and the additional compensating strain gauge, while not subjected to tensile or compressive stress, are nevertheless subjected to transverse contraction as a result of the tensile or compressive stress. For example, at a measuring point made of metal, the transverse contraction factor is 0.3. According to the invention, therefore, not only the output signal of the strain gauges can be used for evaluation, but also the output signal of the compensating strain gauges. Thus, by combining a strain gauge with a compensating strain gauge, a cumulative output signal with a 30% increase in strength can be generated, for example, when metal is used at the measuring point, thereby improving the measurement sensitivity and the signal-to-noise ratio. With suitable interconnection of the strain gauges and compensating strain gauges, the sensitivity can even be increased to 2.6.Preferably, in the embodiment according to the invention, due to the consideration of transverse contraction by the compensation strain gauges, both the strain gauges and the composite strain gauges contribute to the output signal to be evaluated. The compensation strain gauges are used multifunctionally, since they serve on the one hand for temperature compensation and on the other hand contribute to the output signal and measurement signal to be evaluated.

[0021] It is known from the prior art to apply compensating strain gauges to an additional test strip, separate from the actual measuring structure, whose material ideally matches that of the measuring structure and which is exposed to the same temperature changes. This can be problematic because an additional location must be provided for the test strip. Furthermore, it is not always possible to make the material of the test strip identical to that of the measuring structure. In the present case, the compensating strain gauge is applied directly to the primary wing structure, thus eliminating the need for an additional test strip.

[0022] It is also possible that the primary wing structure is designed to measure a stress on the primary wing structure caused by a shear force that induces shear strain in a cross-sectional area of ​​the primary wing structure. A shear bridge with strain gauges, such as those used in [reference to specific examples], is possible in this case. Fig. 7 of the aforementioned publication by Voß, A., Soal, K., Sinske J., Meier, D., Niemann, S., Nickle, J., and Hanke M.

[0023] It is also possible that the primary wing structure is designed to measure stress on the primary wing structure by torsion about its longitudinal axis. For this purpose, a torsion bridge, such as the one described in [reference missing], can be used. Fig. 9 of the publication mentioned at the beginning by Voß, A., Soal, K., Sinske J., Meier, D., Niemann, S., Nickle, J., and Hanke M.

[0024] Another proposal involves more than simply recording one of the aforementioned stresses. Instead, strain gauges and Wheatstone bridge circuits can be incorporated into the primary wing structure, allowing for further measurement. - the recording of a bending stress of the primary wing structure as previously explained and / or - the detection of a stress on the primary airfoil structure with a shear force as previously explained and / or - the detection of a stress on the primary wing structure with a torsion about the longitudinal axis of the primary wing structure can be carried out as previously explained.

[0025] An alternative or cumulative reduction of the influence of temperature changes, particularly those resulting from energy input through solar radiation and / or energy outflow through cold winds, can be achieved if the measuring point and / or the subsequent measuring point (and optionally also the compensation strain gauge and the subsequent compensation strain gauge) are insulated and / or shielded against radiation. Any structures, configurations, and materials can be used for the insulation and shielding. In one embodiment of the invention, the insulation and / or shielding is achieved by means of an insulating and / or shielding body, which is preferably made of foam and / or a film (particularly one that reflects solar radiation).

[0026] Advantageous further developments of the invention result from the patent claims, the description and the drawings.

[0027] The advantages of features and combinations of features mentioned in the description are merely exemplary and can have an effect alternatively or cumulatively, without the advantages necessarily having to be achieved by embodiments according to the invention.

[0028] Regarding the disclosure content—not the scope of protection—of the original application documents and the patent, the following applies: Further features can be derived from the drawings—in particular, the geometries depicted and the relative dimensions of several components to one another, as well as their relative arrangement and functional connection. The combination of features from different embodiments of the invention or from features of different claims is also possible, deviating from the chosen cross-references of the claims, and is hereby encouraged. This also applies to features that are illustrated in separate drawings or mentioned in their description. These features can also be combined with features from different claims.Likewise, features listed in the patent claims may be omitted for further embodiments of the invention, but this does not apply to the independent patent claims of the granted patent.

[0029] The features mentioned in the claims and the description are to be understood, with regard to their number, as meaning that exactly that number or a greater number than the stated number is present, without the need for the explicit use of the adverb "at least". Thus, for example, if an element is mentioned, this is to be understood as meaning that exactly one element, two elements, or more elements are present. The features listed in the claims may be supplemented by further features or may be the only features that the subject matter of the respective claim possesses.

[0030] The reference numerals contained in the patent claims do not constitute a limitation of the scope of the subject matter protected by the patent claims. They merely serve the purpose of making the patent claims easier to understand. BRIEF DESCRIPTION OF THE FIGURES

[0031] The invention will now be further explained and described with reference to preferred embodiments shown in the figures. Fig. Figure 1 shows the temperature profile of a strain gauge, namely - a target temperature profile when the strain gauge is applied to a material with a non-linear measurement error, - the temperature change of the apparent strain when the strain gauge is applied to a material different from the intended material and - the resulting actual temperature profile when the strain gauge is applied to a material different from the intended material. Fig. Figure 2 shows the arrangement of strain gauges on a primary airfoil structure for detecting bending stress. Fig. Figure 3 shows a Wheatstone bridge circuit. Fig. Figure 4 shows a resulting measurement error for the detection of a bending moment when the primary airfoil structure is designed according to Fig. is exposed to 5 time-changing temperatures. FIGURE DESCRIPTION

[0032] Fig. Figure 1 shows temperature profiles of a strain gauge, where a strain 1 [ε in µm / m] measured by the strain gauge is plotted against the temperature 2 [°C].

[0033] The target temperature profile 3 is the temperature profile provided by the strain gauge manufacturer for a strain gauge, which is adapted for a given material such that at a reference temperature 4, in particular room temperature, a local or absolute maximum 5 occurs and the measured strain 1 is exactly zero. It can be seen in the target temperature profile 3 that when the strain gauge is placed on the material being tested, a deviation 6 is measured at a temperature other than the reference temperature 4, which is due to the non-linear behavior of the strain gauge.

[0034] According to the invention, the strain gauge is not mounted on the material of the strain gauge, which has a coefficient of thermal expansion α. B has, glued on, but on a different material with a coefficient of thermal expansion α TIf the primary structure of the wing, made of material with a coefficient of thermal expansion α, heats up... T , onto which the strain gauge is glued, results from the differences in the expansion coefficients α B , α T an apparent strain ε S , which in Fig. 1 is labelled with the reference number 7. The apparent strain 7 is linear with a positive slope (where α is the reference number). B , α T (the slope can also be negative) and crosses the x-axis of the temperature profile at the reference temperature. The magnitude of the apparent strain 7 increases proportionally to the distance from the reference temperature 4. The apparent strain 7 is negative for temperatures lower than the reference temperature 4, while it is positive for temperatures higher than the reference temperature 4.

[0035] As an example, the strain gauge is used for a target temperature 8 of 60°C. It can be seen that at the target temperature 8, the deviation 9 and the apparent strain 10 have opposite signs, so that the apparent strain 10 and the deviation 9 partially cancel each other out. This results in a reduced measurement error 11 at the target temperature (and in its vicinity), particularly at higher temperatures, which is further reduced by Fig. Figure 1 is shown for different target temperatures. The course of the measurement error 11 as a function of temperature is referred to as the actual temperature profile 37 and is shown in Fig. 1. It can be seen that in a temperature range 12, for example from 0°C to 80°C, a reduced measurement error 11 results. It is possible that for temperatures outside the temperature range 12, especially for temperatures below 0°C, the measurement error 11 has a negative value that increases with increasing distance from the reference temperature 4.

[0036] Fig. Figure 2 shows a primary airfoil structure 13, which is only shown schematically here and which can be a tubular primary airfoil structure. The primary airfoil structure 13 is subjected to a bending moment 14, which acts about a bending axis 16 oriented transversely to a longitudinal axis 15 of the primary airfoil structure 13, in particular horizontally.

[0037] Measuring points 17 and 18 are provided on the primary wing structure 13. At least at measuring points 17 and 18, the material of the primary wing structure 13 has a coefficient of thermal expansion α. T Measuring point 17 is located on the upper surface of the primary wing structure 13, and the material of measuring point 17 is subjected to tensile stress in the direction of the longitudinal axis 15 as a result of the bending moment 14. Measuring point 18 is located on the lower surface of the primary wing structure 13, such that the material of measuring point 18 is subjected to compressive stress in the direction of the longitudinal axis 15 as a result of the bending moment 14.

[0038] At measuring point 17, a strain gauge 19 is oriented with its measuring axis parallel to the longitudinal axis 15, so that this strain gauge 19 is subjected to tensile stress. Similarly, at measuring point 18, another strain gauge 20 is arranged, which is oriented parallel to the longitudinal axis 15 and is therefore subjected to compressive stress.

[0039] In measuring points 17 and 18, a compensation strain gauge 21 and another strain gauge 22 are arranged, respectively, wherein the measuring axes of the compensation strain gauges 21, 22 are oriented in the circumferential direction, so that the compensation strain gauges 21, 22 are not subjected to the tension or compression in measuring point 17, 18 as a result of the acting bending moment 14, but may be subjected to a transverse contraction.

[0040] Fig. Figure 3 shows a Wheatstone bridge circuit 23. In the Wheatstone bridge circuit 23, a voltage source 24 is connected to two parallel voltage divider branches 25 and 26. In the voltage divider branch 25, a first resistor 27 and a fourth resistor 28 are connected in series. A bridge junction 29 is arranged between the resistors 27 and 28. Similarly, in the second voltage divider branch 26, a second resistor 30 and a third resistor 31 are connected in series, with the resistors 30 and 31 separated from each other by a second bridge junction 32. The two bridge junctions 29 and 32 are connected to each other by a measuring bridge 33, and the voltage signal measured at the measuring bridge 33 is evaluated as the measurement signal.

[0041] Preferably, in the Wheatstone bridge circuit 23, the following applies: Fig. 3 of the strain gauges 19 the first resistor 27, the next strain gauge 20 the second resistor 30, the compensation strain gauge 21 the fourth resistor 28 and the next compensation strain gauge 22 the third resistor 31.

[0042] Fig. Figure 4 shows the course of a measurement error 34 [voltage signal µV] as a function of time 35 [s] when using a primary airfoil structure according to Fig. 2 and wiring of the strain gauges according to Fig. 3, if a temperature of 36 [°C] is changed over a time of 35 as described in Fig. Figure 5 shows that a very small measurement error occurs, which is particularly the case in temperature range 12. REFERENCE MARK LIST 1 Stretching 2 Temperature 3 Target temperature range 4 Reference temperature 5 Maximum 6 deviations 7 apparent elongation 8 Target temperature 9 Deviation 10 apparent elongation 11 Measurement errors 12 Temperature range 13 Wing primary structure 14 Bending moment 15 Longitudinal axis 16 Bending axis 17 measuring point 18 measuring point 19 strain gauges 20 more strain gauges 21 Compensation strain gauges 22 additional compensation strain gauges 23 Wheatstone bridge circuit 24 Voltage source 25 Voltage divider branch 26 Voltage divider branch 27 first resistance 28 fourth resistance 29 first bridge point 30 second resistor 31 third resistance 32 second bridge point 33 Measuring bridge 34 measurement errors 35 Time 36 Temperature 37 Actual temperature range

Claims

[1] Primary wing structure (13) with a) a measuring point (17; 18) which has a coefficient of thermal expansion α T exhibits, and b) a strain gauge (19; 20) attached to the measuring point (17; 18) of the primary wing structure (13), which has a target temperature profile (3) at a reference temperature (4) and when attached to a material with a coefficient of thermal expansion α B has a maximum (5) and / or is zero, where c) the material being determined differs from the material at the measuring point (17; 18) and the coefficient of thermal expansion α B of the material being determined, the coefficient of thermal expansion α Tthe primary structure of the wing (13) deviates, such that the actual temperature profile (37) of the strain gauge (19; 20) at the reference temperature (4) and when the strain gauge (19; 20) is attached to the measuring point (17; 18) of the primary structure of the wing (13) is not zero and / or does not have a maximum (5), and d) the target temperature range (3) of the strain gauge (19; 20) is adapted to the material of the primary wing structure (13) such that at a target temperature (8) that deviates from the reference temperature (4) da) an apparent strain ε S (10) which result from the different coefficients of thermal expansion α B and α T and the resulting thermal expansions ε T the primary airfoil structure (13) at the target temperature (8) and ε DMS of the DMS at the target temperature (8), and db) at least partially eliminate a deviation (9) of the target temperature rate (3) of the strain gauge (19; 20) at the target temperature (8) from the target temperature rate (3) of the strain gauge (19; 20) at the reference temperature (4). [2] Primary wing structure (13) according to claim 1, wherein the amounts a) the apparent strain ε S (10) which result from the different coefficients of thermal expansion α B and α T and the resulting thermal expansions ε T the primary airfoil structure (13) at the target temperature (8) and ε DMS of the DMS at the target temperature (8), and b) the deviation of the target temperature rate (3) of the strain gauge (19; 20) at the target temperature (8) from the target temperature rate (3) of the strain gauge (19; 20) at the reference temperature (4) differs by a maximum of 50%. [3] Primary wing structure (13) according to one of the preceding claims, wherein the target temperature (8) is in the range of 40°C to 100°C, preferably in the range of 60°C to 90°C or in the range of 70°C to 90°C. [4] Primary wing structure (13) according to one of the preceding claims, wherein the primary wing structure (13) is designed to measure a bending stress of the primary wing structure (13). [5] Primary wing structure (13) according to claim 4, wherein a) the strain gauge (19) is arranged in the area of ​​a measuring point (17) subjected to tensile stress as a result of bending stress, b) a further corresponding strain gauge (20) is arranged in the area of ​​a further measuring point (18) which is subjected to pressure as a result of the bending stress, c) a Wheatstone bridge circuit (23) is present ca) with two parallel voltage divider branches (25, 26), wherein in a first voltage divider branch (25) a first resistor (27) and a fourth resistor (28) are arranged in a series circuit, between which a first bridge point (29) is arranged, and in a second voltage divider branch (26) a second resistor (30) and a third resistor (31) are arranged in a series circuit, between which a second bridge point (32) is arranged, and cb) with a measuring bridge (33) which connects the two bridge points (29, 32) and d) the strain gauge (19) forms the first resistance (27) and the further strain gauge (20) forms the second resistance (30), wherein the first resistance (27) and the second resistance (30) are arranged in the two voltage divider branches (25, 26) on the same sides of the bridge points (29, 32). [6] Primary wing structure (13) according to claim 5, wherein a) a compensation strain gauge (21) is present, which is arranged in the area of ​​the measuring point (17), but is oriented such that it is not subjected to tension or compression as a result of the bending stress of the primary structure of the airfoil (13), wherein the compensation strain gauge (21) forms the fourth resistor (28), and b) a further compensating strain gauge (22) is present, which is arranged in the area of ​​the further measuring point (18), but is oriented in such a way that it is not subjected to tension or compression as a result of the bending stress of the primary structure of the airfoil (13), wherein the further compensating strain gauge (22) forms the third resistance (31). [7] Primary wing structure (13) according to any one of claims 1 to 3, wherein the primary wing structure (13) is designed to measure a stress on the primary wing structure (13) with a shear force. [8] Wing primary structure (13) according to any one of claims 1 to 3, wherein the wing primary structure (13) is designed to measure a stress on the wing primary structure (13) with a torsion about a longitudinal axis (15) of the wing primary structure (13). [9] Primary wing structure (13) with strain gauges (19, 20) and Wheatstone bridge circuits (23) for measurements a) a bending stress of the primary wing structure (13) according to one of claims 4 to 6 and / or b) a stress on the primary wing structure (13) with a shear force according to claim 7 and / or c) a stress on the primary wing structure (13) with a torsion about the longitudinal axis of the primary wing structure (13) according to claim 8. [10] Primary wing structure (13) according to one of the preceding claims, wherein the measuring point (17) and / or the further measuring point (18) are insulated and / or shielded against radiation. [11] Primary wing structure (13) according to one of the preceding claims, wherein the insulation and / or shielding is provided by means of an insulation and / or shielding body which preferably comprises foam and / or a film.

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